The marine industry seeks to provide a multitude of fish products by growing fish in a controlled environment. The industry is currently experiencing rapid growth, resulting in a many different types of equipment that are necessary to nurture and harvest fish. When compared to the conventional techniques that are employed by most commercial fishing operations to harvest wild fish, the advantages of marine aquaculture are several, among them are predictable yields in terms of the number of fish harvested, as well as reductions in labor and equipment costs. This is a welcome development both from the standpoint of profitability and meeting the global demand for seafood.
The typical marine aquaculture enclosure has a weighted, polymer fiber mesh net formed into a rectangular, square or round cage that is suspended in a water body by attached flotation devices. The cage contains the fish for a period of months. For example, farm-raised salmon spend about 18 months enclosed in cages. In addition to containing the fish for easy feeding and harvesting, the cage provides protection from aquatic predators such as seals and sea lions. At the end of a given growth period, the fish crop is removed from the cage.
Metallic cages, typically constructed of galvanized steel or special anti-fouling copper alloy wires, are also used in marine aquaculture. The service lifetime of metallic wire nets is limited primarily by mechanical wear, surface corrosion, and fretting corrosion. Wear, leading to holes in the net, is caused by the relative motion of opposing surfaces due to movement of the net as a consequence of wave and water currents or by the repetitive movement of fish against the net. Corrosion of metallic nets reduces the thickness of the net and can lead to failure of the net and escape of the fish. Corrosion significantly shortens the service life of the cage.
In practice, floating structures support a cage constructed of fabricated metal wire that may be woven into netting and assembled to form an enclosure in which fish are contained. Certain alloys, such as those containing a high percentage of copper, also provide resistance to biofouling. This combination of properties provides an advantageous enclosure for fish in seawater environments. However, traditional metal cages are subject to corrosion in seawater that reduces the thickness of the metal net, and this limits the useful life of the cage. Further, corrosion occurs at a significantly higher rate in the top portion of a cage than the continuously submerged portion of the cage. This increased corrosion rate in the surf zone is well known and occurs where the cage is exposed to highly aerated seawater, higher current velocities, and mechanical motion that removes protective oxide films that form on certain metal surfaces, such as copper. High corrosion rates are seen typically within the top one meter of the net. Corrosion rates below one meter diminish significantly.
The corrosive action of sea water consumes and reduces the thickness of the metal nets, thereby limiting the useful life of the cage. It is not always practical, however, to increase the thickness of the metal used in the net to increase service life because this would significantly increase the weight of the net and the size and cost of the flotation system. A typical cage for large scale fish culture can have dimensions of 30 m length by 30 m width by 15 m depth and contain up to 20 tons of metal wire of 4 mm diameter and an opening of 40 mm in the netting. This amount of material can be difficult to handle and requires large floats. This increase in weight places heavy demands on the net flotation and mooring systems.
To address the concerns of corrosion, cages have been developed from synthetic materials such as nylon, plastic, and other polymers. Synthetic cages produce a host of other issues, however. The synthetic materials, including synthetic materials with known antifouling coatings, are susceptible to biofouling, which refers to an accumulation on the net of marine organisms, including parasites and other pathogens that are harmful to the fish being cultivated. The presence of these harmful organisms can lead to diseased fish, requiring the use or increased use of antibiotics or other methods in an attempt to keep the fish healthy. In addition, fouling decreases the flow of clean oxygenated water into the cage which can adversely affect fish health and growth rate.
Thus, there is a need in the art for an aquaculture cage that resists biofouling, provides protection from predators and has improved longevity.